Thermally stabilized waveguides

ABSTRACT

An apparatus comprising a planar optical waveguide having an optical core and optical cladding next to the optical core. The optical core or cladding includes a plurality of particles therein. Each particle has a nucleus and polymeric molecules permanently bonded to the nucleus to form a shell. A plurality of nuclei are dispersed in said core or cladding.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to planar opticalwaveguides.

BACKGROUND OF THE INVENTION

Planar optical waveguides enjoy widespread use in optical communicationstechnology, including switches, filters and multiplexers. Stability ofthe index of refraction, n_(i), of the waveguide material is desirableto maintain device operation within a desired tolerance. However, then_(i) of such materials commonly varies as a function of temperature.Such variability is commonly defined by a nonzero value of thethermo-optic coefficient (TOC), or ∂n/∂T.

In many cases, required performance stability of an optical systemnecessitates controlling the temperature of a waveguide to maintain then_(i) of the waveguide within design tolerances. Such temperaturecontrol imposes costs and complexity on system design and manufacture.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, variousembodiments provide an apparatus having planar optical waveguides (POWs)that may have reduced thermo-optic coefficients.

One embodiment is an apparatus that includes a POW having an opticalcore and optical cladding next to the optical core. The optical core orcladding includes a plurality of particles therein. Each particle has anucleus and polymeric molecules permanently bonded thereto to form apolymer shell. A plurality of nuclei are dispersed in the core orcladding.

Another embodiment is a method of fabricating a planar opticalwaveguide. The method includes chemically bonding a polymer to a surfaceof a nucleus to form a particle having a polymer shell. A layer of theparticles is formed on a substrate, where the layer includes a pluralityof nuclei dispersed in the polymer. Portions of the layer areselectively removed to form a planar optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the invention, reference is nowmade to the following descriptions taken in

FIG. 1 illustrates a planar optical waveguide (POW);

FIGS. 2A through 2C illustrate a nucleus with a polymer shell;

FIG. 3 illustrates embodiments of methods of producing a POW includingnuclei with polymer shells dispersed therein;

FIGS. 4A and 4B illustrate formation of poly(dimethyl siloxane), andattachment thereof to a nucleus;

FIGS. 5A through 5H illustrate sectional views of a POW at variousstages of formation; and

FIG. 6 illustrates an apparatus employing a POW.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of a planar optical waveguide (POW)100. The POW 100 includes a substrate 110 with an optical core 120thereon. The core 120 is substantially transparent to a wavelength oflight to be used in operation of the POW 100. The substrate 110 may beany substrate with suitable optical properties, e.g., having arefractive index (n_(i)) that is equal to or lower than the refractiveindex of the core 120.

The POW 100 also may include an optical cladding 130. In one aspect, theoptical cladding also has a refractive index less than that of the core120. In some cases the cladding 130 may be omitted, as the refractiveindex of air will be less than the refractive index of the core 120.

At least one of the core 120 and the cladding 130 includes nuclei 140substantially uniformly dispersed therein. In an embodiment, the nuclei140 include a material that is transparent to a wavelength of lightconfined by the POW 100. The core 120 or cladding 130 also includes amedium 150 that includes a polymer. In one aspect, the nuclei 140 arechemically bonded to the polymer. In another aspect, a nucleus 140 has asurface that is capable of forming a chemical bond with a functionalgroup of a polymer, a “primer” compound or a polymerization initiator.These aspects are discussed further below. Materials suitable for use asthe nuclei 140 include, but are not limited to, silica, doped silica,fluorozirconate, fluoroaluminate, and chalcogenide glasses, and polymerssuch as poly(dimethyl siloxane) (PDMS) and derivatives thereof.Functional groups that may be used to attach a primer compound or apolymerization initiator to the nuclei 140 include chlorosilanes(—SiR_(n)Cl_(3-n), where n=0-2), alkoxysilanes (—SiR_(n)OR_(3-n), wheren=0-2), carboxylates (—COOH), and phosphonates (—PO₃R₂).

The polymer included in the medium 150 has suitable properties for usein optical waveguides. Suitable characteristics include opticaltransparency at the operating wavelength of the light, dimensionalstability and low optical loss at telecommunications wavelengths.Non-limiting examples of such polymers include poly(dimethyl siloxane)(PDMS), poly(acrylate), poly(carbonate), poly(styrene), and derivativesthereof.

FIG. 2A illustrates a conceptual view of a particle 200 that includes anucleus 210 and a polymer shell 215. In some embodiments, bondedpolymers are believed by the inventors to form a substantiallycontinuous coating on a surface 220 of the nucleus 210. When a layerincluding a plurality of particles 200 is formed, the polymers arebelieved to intermix to form the medium 150 in which the nuclei 140 aresubstantially homogeneously distributed. When the polymers aresufficiently long and flexible, they may additionally entangle. Forexample, in some cases, polymers may entangle when they include overabout 200 monomer repeat units. As described below, the layer may thenbe formed into the POW 100.

The polymer may have a functional group capable of directly bonding tothe surface 220 of the nucleus 210. In such cases, the functional groupmay be at a terminal or non-terminal position on the polymer chain. Thecomposition of such a functional group may depend on the chemicalcomposition of the nucleus 210. When the nucleus 210 comprises silica,e.g., the functional group may include silicon, and may further includean alkoxysilane moiety.

A primer or initiator compound may also have a functional group capableof bonding to the surface 220 of the nucleus 210. Again, for the examplein which the nucleus 210 comprises silica, this functional group mayinclude silicon, and may further include an alkoxysilane moiety. In somecases, the primer or initiator may include a derivative of amono-trichlorosilane or a mono-trialkoxysilane. The primer additionallyhas a functional group capable of bonding to a reactive site on apolymer in the medium 150. The reactive site may be at a terminal ornon-terminal position on the polymer chain.

The initiator has a functional group capable of initiatingpolymerization of a monomer. In some cases, the initiator includes amoiety capable of initiating anionic polymerization. Such a moiety mayinclude, e.g., a lithium atom complexed with an organic anion. In othercases, the initiator includes a moiety capable of initiating atomictransfer radical polymerization (ATRP). Such a moiety may include, e.g.,an alpha-bromo-substituted carbonyl such as the alpha-bromo-isobutyrylgroup.

The nucleus 210 and the shell 215 have a bulk thermo-optic coefficient(TOC), ∂n/∂T. A bulk TOC is the TOC exhibited by a sample of a materiallarge enough that the presence of the surface has a negligible effect onthe TOC of the sample. The TOC may be approximately constant within arange of temperatures. Furthermore, the TOC may be greater than zero,indicating an increase of the index of refraction with increasingtemperature, or less than zero, indicating a decrease of the index ofrefraction with increasing temperature. When the term thermo-opticcoefficient or TOC is used herein, unless otherwise stated it isunderstood to refer to the thermo-optic coefficient at room temperature,about 25° C.

Advantageously, a nucleus 210 having a TOC of one sign may be combinedwith a shell 215 having the opposite sign to form a core 120 or cladding130 having a desired TOC. For example, silica typically has a TOC ofabout +1 E-5° C.⁻¹, while PDMS may have a TOC ranging from about −1.5E-4° C.⁻¹ to about −5 E-4° C.⁻¹. In one aspect, the TOC of the nucleus210 and the shell 215 may be chosen to result in a desired TOC of thePOW 100. In some embodiments, the desired TOC may be approximately zero.For example, as described below, the ratio of volume of the nucleus 210and the shell 215 may be chosen to result in a TOC of the POW 100 of 1E-6° C.⁻¹, 1 E-7° C.⁻¹, or less. Note that if the temperature rangewithin which the TOC of the nucleus 210 and shell 215 are constant islimited, the temperature range within which the TOC retains its targetvalue may be similarly limited.

The nucleus 210 is shown as spherical for illustration, but may be anygeometry that does not result in significant scattering of light by thenucleus 210 at the optical wavelength of interest. The nucleus 210 mayscatter an optical wave when the size of the nucleus 210 exceeds aminimum fraction of the wavelength λ of the wave. To reduce suchscattering, it is preferred that no linear dimension of the nucleus 210exceeds about ½λ. More preferably, no linear dimension of the nucleus210 exceeds about ¼λ. Most preferably, no linear dimension of thenucleus 210 exceeds about 1/10λ. In one embodiment, if a POW 100 is usedto propagate an optical signal in the C-band (about 1525-1565 nm), apreferred upper limit on a diameter of the nucleus 210 would be about150 nm or less. Furthermore, as discussed below, there may be apreferred lower limit on size of the nucleus 210.

Turning to FIG. 2B, a detailed conceptual view of the particle 200 isillustrated. In the illustrated embodiment, the shell 215 is formed frompolymer emulsion particles 225 bonded to the surface 220 of the nucleus210. Each emulsion particle 225 includes one or more polymer chains 230.A primer 235 includes functional groups R and R′. The functional group Ris bonded to the surface 220. The polymer chain 230 is bonded to thefunctional group R′, thus attaching the emulsion particle 225 to thenucleus 210.

The emulsion particles 225 may be formed with a desired diameterdetermined by the conditions of formation. The shell 215 is expected tohave about the same thickness as the diameter of the emulsion particles225. For example, if the average diameter of the emulsion particles 225is about 5 nm, the average thickness of the shell 215 may be about 5 nm.

In a second embodiment, illustrated in FIG. 2C, the particle 200includes a polymerization initiator 240 attached to the surface 220 viaa functional group R″. As described in greater detail below, theinitiator 240 is first bonded to the surface 220. Then, a solution ofmonomers is provided to form the polymer chain 245 from an initiatorgroup R′″. The thickness of the resulting polymer shell 215 may becontrolled by constraining the length of the polymer chains 245 formedon the surface 220. For example, the polymer chain 245 may be formedwith a desired chain length by constraining the amount of monomeravailable. After reaching the desired length, the polymer chain 245 maythen be terminated with a suitable end group. The thickness of the shell215 is expected to be related to the length of the polymer chains formedon the surface 220. However, the thickness is expected to typically beless than the length of the polymer chains due to entropic effects andcohesion among neighboring polymer chains.

In a third embodiment (not shown), polymer emulsion particles are bondeddirectly to the nucleus 210. In this case, a functional group capable ofbonding to the surface 220 is present on the polymer chain prior tobonding the polymer chain 230 to the surface 220. The functional groupmay contain silicon when the nucleus 210 comprises silica, and mayfurther include an alkoxysilane moiety.

The TOC of the core 120 and the cladding 130 may be advantageouslyadjusted by controlling the volume fraction of the nuclei 140 and themedium 150 therein. In general, the TOC is expected to be approximatelythe sum of each TOC of the nuclei 140 and the medium 150, each weightedby its respective volume fraction. In some embodiments, the TOC valuetargeted is less than a maximum design value. This relationship may beexpressed asVF _(Nuclei) *TOC _(Nuclei) +VF _(Medium) *TOC _(Medium)≦Maximum DesignValue,where VF is the volume fraction of the nuclei or the medium.

In cases in which an application is relatively insensitive to the TOC ofthe waveguide, a relatively undemanding requirement may be imposed onthe maximum design value. In some cases, this value may be greater than1 E-6° C.⁻¹. In other cases, a more sensitive application may require amaximum design value less than about 1 E-6° C.⁻¹, while even moresensitive applications may require a maximum design value less thanabout 5 E-7° C.⁻¹. The required volume fraction of the nuclei and themedium may, in general, depend on the TOC of the nuclei and medium andthe targeted maximum design value.

A non-limiting example is provided using the geometry illustrated inFIG. 2A. In this example, imperfect packing of the emulsion particles225 is neglected. Thus, as illustrated in the embodiment of FIG. 2B,about 100% of the surface 220 may be covered by the emulsion particles225. The thickness of the shell 215 may then be approximated by thethickness of the emulsion particles 225. The volume fraction of theshell 215 is then calculated using the diameter of the nucleus 210 andthe thickness of the shell 215. The TOC of silica, about 1 E-5° C.⁻¹,and PDMS, about −1 E-4° C.⁻¹, are used as representative values of theTOC of the nucleus 210 and the shell 215, respectively.

For this case, a volume fraction of about 9% of the shell 215 in thecore 120 or cladding 130 results in a TOC of about 1 E-7° C.⁻. Thisvolume fraction corresponds to a thickness of the shell 215 of about 2nm when the diameter of the nucleus 210 is about 100 nm. In some cases,a larger nucleus 210 may provide better control of the volume fractionof the shell 215. In one embodiment, the diameter of the nucleus 210 isabout 200 nm, and the thickness of the shell is about 3.2 nm. In anotherembodiment, the diameter of the nucleus 210 is about 400 nm, and thethickness of the shell is about 6 nm. In such embodiments, the diameterof the nucleus 210 may be chosen to be below a minimum diameter at whichlight is scattered in the POW 100.

Moving now to FIG. 3, illustrated are embodiments of a method 300 offabricating a planar optical waveguide. The method includes chemicallybonding a polymer to a surface of a nucleus to form a particle having apolymer shell. A layer of the particles is formed on a substrate, wherethe layer includes a plurality of nuclei dispersed in the polymer.Portions of the layer are selectively removed, e.g., via a conventionaletching process, to form a planar optical waveguide.

In a step 305, nuclei for the particles are provided. In some cases, thenuclei are silica particles. In one aspect, silica particles with anaverage diameter ranging from about 100 nm to about 750 nm may be used.Methods of producing silica particles with diameters in this range arewell known to those skilled in the pertinent art.

Steps 310-315-320 and 330-340 represent alternate embodiments of themethod 300. In the embodiment represented by steps 310-315-320, a primercompound is bonded to the surfaces of the nuclei. The primer is acompound having a functional group capable of permanently bonding to thesurface, and a functional group capable of permanently bonding with apolymer emulsion particle. A permanent bond is one that is stable over atemperature range normally experienced by the nucleus, includingtemperatures during the processing, storage and operation of a deviceincluding the nuclei. In some cases the permanent bond is a covalentbond.

In the step 310, a dispersion of nuclei coated with the primer isformed. As a non-limiting example, vinyl trimethoxysilane (TMS) may beused as the primer when the nucleus comprises silica. Vinyltrimethoxysilane may be bonded to the surface of silica particles byheating the silica particles in liquid vinyl TMS at about 100° C. forabout 12 hours with agitation. Under these conditions, it is thoughtthat the TMS moiety forms a silyl ether linkage with a hydroxyl group ona silica particle surface.

In a step 315, a dispersion is formed of polymer emulsion particleshaving a desired diameter. While recognizing that any of the polymerspreviously recited may be used, the case in which the polymer includes aderivative of PDMS is presented as a non-limiting example.

The figure below illustrates an example reaction forming PDMS byanhydrous anionic polymerization of a monomer. In general, a suitablesiloxane monomer may be reacted in the presence of an anionic initiatorsuch as an alkyl lithium compound. In the figure below, the siloxanemonomer is hexamethylcyclotrisiloxane (D3), though other monomers suchas octamethylcyclotetrasiloxane (D4) may also be used. Derivatives ofPDMS may be used to provide sites on the polymer to bond to the primercompound. For example, pentamethylcyclotrisiloxane may be included withthe monomer to provide reactive sites on the polymer chain. In othercases, the PDMS chain may be terminated with a silyl moiety (—SiH₃,—SiH₂R, —SiHR₁R₂ or —SiR₁R₂R₃). Functionalized monomer providing forcross-linking of the polymer may also optionally be included.

The emulsion particles may be formed by emulsion polymerization. In thismethod, polymerization of the monomer occurs in micelles formed bysurfactant molecules. The size of emulsion particles produced may belimited by factors including the amount and type of surfactant used, andthe amount of monomer present. One skilled in the chemical arts maydetermine conditions to produce a desired size of the emulsionparticles.

In step 320, the emulsion particles are attached to the primed nuclei.In one embodiment, the nuclei are suspended in tetrahydrofuran (THF) andheated to reflux (about 64° C.). The suspension of emulsion particles,prepared with suitable functionalization, is added. In the example inwhich the primer is vinyl TMS, the mixture may be maintained at thereflux temperature for about 12 hours to form a polymeric shell.

It is believed that in some cases a hydrosilylation reaction forms abond between PDMS and a nucleus primed with vinyl TMS. Turning to FIG.4A, an example of a potential hydrosilylation reaction is illustrated.The primed nucleus, prepared as described above, is combined withsilane-terminated PDMS in the presence of a Pt catalyst. Karstedt'ssolution of Speirs' hydrosilylation catalyst may be used as the reactionmedium. The mixture may be heated to 80° C. for about 2 hours to drivethe hydrosilylation reaction substantially to completion. The reactionis thought to add the silyl group across the vinyl bond of the vinylTMS, resulting in a bond between the terminal silicon atom and a carbonatom of the vinyl moiety. In some cases, a silyl group may be added to anon-terminal chain position to provide a point of attachment to thesurface. In other cases, a primer compound other than vinyl TMS having aC—C double bond may be used to bond the PDMS chain to the surface.

Turning back to the embodiment in FIG. 3 of the method 300 representedby steps 330-340, a polymerization initiator is attached to the nucleus,and a polymer is grown directly from the initiator. In the step 330, theinitiator is attached to the nucleus. In some cases, the initiator isattached directly to the surface, while in other cases, another compoundmay serve as an intermediate.

Turning briefly to FIG. 4B, illustrated is an embodiment in which ananionic initiator is bonded to a nucleus using an intermediate compoundcomprising a moiety that includes silicon. The intermediate compound isthen reacted with an alkyl lithium compound to form the initiator. Theintermediate compound illustrated is a 1,1-diphenyl, 2-(trialkoxysilyl)ethene as a non-limiting example. The presence of two phenyl rings isthought to stabilize formation of an anion of the attached intermediatecompound when the alkyl lithium compound reacts therewith. However,other intermediate compounds may be used that have more, fewer or nophenyl rings. The moieties shown generally as “R” in FIG. 4B may be anygroup that does not sterically hinder the formation of a bond thatattaches the intermediate compound to the surface 220. In some cases, Rmay be a short chain alkyl group, such as a methyl group.

Butyl lithium is illustrated as a non-limiting example of an alkyllithium compound. Under anhydrous conditions, a butyl anion may reactwith the ethene carbon-carbon double bond. The reaction in this exampleforms the butyl derivative of the intermediate compound. The derivativehas a stabilized negative charge that may initiate polymerization of amonomer.

Returning to FIG. 3, in the step 340, the nuclei with the attachedanionic initiator are combined with a monomer. The monomer is reactedwith the nuclei to produce coated nuclei. The quantity of monomer may belimited to result in a shell having a desired thickness. After thedesired thickness is produced, the polymer chains may be terminated.

Turning briefly again to FIG. 4B, illustrated is the case in which themonomer is D3. The anion produced by attaching the alkyl lithiumderivative to the nucleus is used to initiate polymerization of themonomer under anhydrous conditions. In some cases, a functionalizedmonomer may optionally be included in the monomer stock to providecross-linking of the resulting polymer at a later stage of manufacture.

The following non-limiting example describes a method of forming thepolymer on the surface of the nucleus. Nuclei are prepared with ananionic initiator primer as described previously. The nuclei aredispersed in cyclohexane in a reaction vessel from which water has beenpurged. A dry nitrogen atmosphere is maintained to exclude water. Atroom temperature, butyl lithium is added with stirring to the dispersionto convert the attached primer to the anion. The solution is permittedto stir for about 30 minutes to ensure complete reaction. D3 monomer isthen added and the solution is allowed to react with agitation for about3 hours. The coated nuclei may be precipitated from the solution by theaddition of methanol.

Returning to FIG. 3, in the step 345, the coated nuclei are separatedfrom the reaction medium. This may be done, for example, bycentrifugation followed by rinsing with methanol. In a step 350, therinsed coated nuclei are suspended in a suitable carrier solvent. Thecarrier solvent may be any solvent in which the polymer shell is atleast partially soluble, and that is compatible with the method used toform a layer of coated nuclei on a substrate. When the polymer shell isa derivative of PDMS, THF may be a suitable carrier solvent.

In the following description of subsequent steps in the method 300,reference is also made to FIGS. 5A-5H showing sectional views of a POWin various stages of manufacturing. In a step 360, a substrate 505,illustrated in FIG. 5A, is provided. As used herein, a substrate isprovided when it is obtained from any source or formed by any currentlyexisting or future discovered method. In FIG. 5B, a layer 510 includingthe coated nuclei is formed on the substrate 505. Methods of determiningthe specific coating conditions to result in a desired thickness anduniformity of the resulting layer are well known to those skilled in thepertinent art. The polymer may optionally be cross-linked at this point.A process utilizing short-wavelength light (ultraviolet, e.g.) orthermal activation may be used as appropriate.

In a step 370, the layer 510 is patterned to form a POW core. In FIG.5C, a layer of photoresist 515 is formed over the layer 510. In FIG. 5D,the photoresist is patterned to form a mask layer 520. In FIG. 5E,exposed portions of the layer 510 are removed by plasma etch, e.g., toform a POW core 525, and the mask layer 520 is removed. Methods ofachieving these recited steps are known to those skilled in thepertinent arts. If additional cross-linking is desired or cross-linkingwas not previously performed, the polymer in the POW core 525 mayoptionally be cross-linked at this point.

In a step 380, a cladding layer 530, illustrated in FIG. 5F, is formedover the POW core 525. The cladding layer 530 may be a conventionalmaterial suitable for use with the POW core 525, or may be a compositionformed as described herein. Restated, the cladding layer 530 may alsoinclude nuclei having a polymer chemically bonded thereto, but designedto have a lower index refraction than the POW core 525. The polymer inthe cladding layer 530 may optionally be cross-linked at this point.

In a step 390, the cladding layer 530 is patterned to form a cladding540. FIG. 5G illustrates a mask layer 535 formed by processing aphotoresist layer (not shown) by conventional lithographic techniques.FIG. 5H shows a cladding 540 after selective removal of exposed portionsof the cladding layer 530 by plasma etch, e.g., and removal of the masklayer 535. If not already performed, the polymer in the POW core 525 orthe cladding 540 may optionally be cross-linked at this point.

Turning now to FIG. 6, illustrated is an example apparatus 600 using thePOW 100 described herein. The apparatus 600 is shown as a Mach-Zehndermodulator as a non-limiting example. An input optical signal 610 isprovided to the apparatus 600. The input optical signal 610 is splitbetween waveguide arms 620, 630 and recombined to form an output opticalsignal 640. Driving electronics 650 provide control signals to result ina desired modulation of the optical signals passing through thewaveguide arms 620, 630. Those skilled in the optical arts willappreciate that the POW 100 may be used with any optical apparatusrequiring a planar optical waveguide, such as switches, filters andmultiplexers.

Although the present invention has been described in detail, thoseskilled in the pertinent art should understand that they can makevarious changes, substitutions and alterations herein without departingfrom the spirit and scope of the invention in its broadest form.

1. An apparatus, comprising: a planar optical waveguide having anoptical core and optical cladding next to said optical core, saidoptical core or cladding including a plurality of particles therein; andwherein each particle has a nucleus and polymeric molecules permanentlybonded to said nucleus to form a polymer shell, and a plurality ofnuclei are dispersed in said core or cladding, and said waveguide isconfigured to operate in the optical C-band.
 2. The apparatus as recitedin claim 1, wherein a bulk thermo-optic coefficient (TOC) of saidnucleus and a bulk TOC of said shell have opposite signs.
 3. Theapparatus as recited in claim 2, wherein a volume fraction of saidnucleus and a volume fraction of said shell in said optical core orcladding are selected to result in about equal and oppositecontributions to said TOC of said waveguide by said volume fractions. 4.The apparatus as recited in claim 3, wherein a sum of a product of saidvolume fraction of said nucleus multiplied by said TOC of said nucleusand a product of said volume fraction of said shell multiplied by saidTOC of said shell is less than about 1 E-6° C.⁻¹.
 5. The apparatus asrecited in claim 1, wherein said nucleus comprises silicon dioxide. 6.The apparatus as recited in claim 1, wherein said polymeric moleculesare selected from the group consisting of: a derivative of poly(dimethylsiloxane); a derivative of poly(acrylate); a derivative ofpoly(carbonate); and a derivative of poly(styrene).
 7. The apparatus asrecited in claim 1, wherein said polymeric molecules are bonded to saidnucleus by a covalent bond.
 8. The apparatus as recited in claim 1,wherein said nucleus has a dimension ranging from about 100 nm to about750 nm.
 9. The apparatus as recited in claim 1, further comprising aswitch, a filter or a multiplexer, wherein said switch, filter ormultiplexer includes said planar optical waveguide.
 10. An apparatus,comprising: a planar optical waveguide having an optical core andoptical cladding next to said optical core, said optical core orcladding including a plurality of particles therein, wherein eachparticle has a nucleus and polymeric molecules permanently bonded tosaid nucleus to form a polymer shell; a plurality of nuclei aredispersed in said core or cladding; a bulk thermo-optic coefficient(TOC) of said nucleus and a bulk TOC of said shell have opposite signs;and a volume fraction of said nucleus and a volume fraction of saidshell in said optical core or cladding are selected to result in aboutequal and opposite contributions to said TOC of said waveguide by saidvolume fractions.
 11. An apparatus, comprising: a planar opticalwaveguide having an optical core and optical cladding next to saidoptical core, said optical core or cladding including a plurality ofparticles therein, wherein each particle has a nucleus and polymericmolecules permanently bonded to said nucleus to form a polymer shell; aplurality of nuclei are dispersed in said core or cladding; and saidpolymeric molecules are bonded to said nucleus by a covalent bond.